Known communications systems and networks allow wireless devices to communicate information over radio frequency (RF) channels. Such wireless systems, networks, and devices offer many advantages in terms of making information ubiquitously available and easily accessible in various business and personal applications. Recognizing the benefits associated with wireless communication of information, governmental agencies responsible for frequency spectrum allocation have allocated specific frequency bands for such use. For example, in the US, the Federal Communications Commission (FCC) in a 1985 ruling made portions of the spectrum previously designated for military use, namely, in the 915 MHz, 2.4 GHz bands available and designated them as Industrial, Scientific and Medical (ISM) bands for communicating information without a license. In addition, the FCC allocated portions of the 5 GHz spectrum as the Unlicensed National Information Infrastructure (UNII) band. Making these portions of the frequency spectrum available for unlicensed use has resulted in proliferation of many types of wireless communication devices ranging from wireless telephones to devices that operate within wireless local-area networks (WLANs). Of course, the unlicensed use of the allocated portions of the spectrum remains subject to FCC regulation, which specifies the rules governing the operational requirements within the allocated bands. The operational requirements are generally defined in terms of allocated spectra and transmission or emission power levels.
Based on the rules promulgated by designated governmental agencies, standardization bodies throughout the world, including ANSI, IEEE, and ISO, have developed various standards that specify the requirements for ensuring interoperability amongst devices offered by different manufacturers. For example, the ANSI/IEEE 802.11 standard defines a protocol for compatible interconnection of data communication equipment via the air, radio, or infrared at specified data rates in local area networks, using a carrier sense multiple access protocol with collision avoidance (CSMA/CA) medium sharing mechanism. The IEEE 802.11 standard, which is hereby incorporated by reference, also specifies a medium access control (MAC) layer that supports operations under control of an access point as well as between independent remote stations. Amongst the functions performed by the 802.11-specified MAC layer are authentication, association, re-association services, encryption/decryption procedures, power management, and point coordination functions for power coordination.
Other standards known as IEEE 802.11a and IEEE 802.11b, which are also hereby incorporated by reference, define higher speed physical (PHY) layer extensions in the 5 GHz UNII band and 2.4 GHz ISM band, respectively. The respective PHY layer standards specify the interface with the 802.11 MAC layer as well as modulation techniques for transmitting information within the allocated bands.
The provisions of the IEEE 802.11, 802.11a and 802.11b standards have now become the blueprints for building local, regional and even nationwide WLANs. These types of WLANs are proliferating to provide high-speed wireless internet access in such places as airports, hospitals, coffee shops, etc. Each WLAN provides wireless communications among a plurality of devices, including an access point as well as remote stations that are situated within a common area. Generally, each mobile and access point device, which functions as an interface between a wireless and a wired network, is equipped with a network interface card (NIC) that incorporates MAC and PHY layer circuitry as well as radio frequency circuitry, including antenna circuitry. Other standards at various stages of development include the 802.11 g and HiperLAN/2 standards that provide additional features such as higher data rates and wireless voice over IP capabilities. Also defined are provisions to provide capabilities amongst mobile wireless stations operating within a plurality of adjacent wireless LANs through access points roaming that provide communication coverage for designated areas. This proliferation of wireless devices that operate in the allocated bands and coexistence among the many wireless systems and devices has placed substantial burden on the available frequency bands in terms of congestion and interference.
Bandwidth availability has therefore become an important factor in proliferation and use of wireless devices. Historically, bandwidth availability, which is regulated by governmental agencies, has been constrained primarily because of technological factors. Data rate throughput is an important parameter in the design of wireless systems. Data rate throughput capability varies proportionally with available bandwidth but only logarithmically with the available signal to noise ratio (SNR). To achieve high capacity data rate systems within a constrained bandwidth, complex signal modulations techniques have been used. Unfortunately, use of such modulation techniques may significantly decrease SNR.
Typically in such systems, data rate is lowered, often dynamically, for example, based on a received signal quality criterion, to improve communication reliability. Conversely, high data rates are used with reduced reliability. The principle barrier to high data rate communications in a wireless local-area network is an interference phenomenon called “multipath.” A radio signal commonly traverses many paths as it travels toward a receiver. Multiple propagation paths can be caused by reflections from surfaces in the environment, for example. Some of these paths are longer than others. Therefore, since each version of the signal travels at the same speed, some versions of the signal will arrive after other versions of the signal. Sometimes the delayed signals will interfere with more prompt signals as the delayed signals arrive at the receiver, causing signal degradation.
Recent advances in communications technology have enabled ultra-wideband (UWB) systems, which can be used for communications, radar, and/or positioning. UWB technology holds great promise for a vast array of new applications that provide significant benefits for public safety, business, and consumers. UWB technology is often referred to as impulse radio technology but may employ any of several types of RF waveforms. Some UWB systems and devices operate by employing very narrow or short duration pulses having a small number of cycles (e.g., one or two cycles) that result in transmissions having very large bandwidths on the order of, for example, several GHz. Narrower bandwidth UWB implementations typically involve wider pulses having many cycles (e.g., 25 to 100) that may have bandwidths on the order of, for example, 500 MHz. Generally, with UWB systems, the shorter the pulse duration the larger the bandwidth, and vice versa.
UWB transmitters and receivers can employ numerous data modulation (and demodulation) techniques, including amplitude modulation, phase modulation, frequency modulation, pulse-position modulation (PPM) and M-ary versions of these (e.g., bi-phase, quad-phase, and M-phase modulation).
Various implementations of impulse radio are described in U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,743,906 (issued May, 10, 1988), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990), U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994), U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997), U.S. Pat. No. 5,812,081 (issued Sep. 22, 1998), U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998), U.S. Pat. No. 5,952,956 (issued Sep. 14, 1999), and U.S. Pat. No. 6,133,876 (issued Oct. 17, 2000), and U.S. patent application Ser. No. 09/811,326 (filed Jul. 20, 2001), U.S. Ser. No. 10/206,648 (filed Jul. 26, 2002), U.S. Ser. No. 60/451,538 (filed Mar. 3, 2003), and U.S. Ser. No. 10/436,646 (filed May 13, 2003), all of which are assigned to the assignee of the present invention and are incorporated herein by reference.
It has been recognized, however, that the benefits of UWB technology could be outweighed by its potential to cause harmful interference, particularly with other important radio operations, such as licensed services. It has also been shown that, under suitable operating restrictions, UWB devices may operate using portions of the frequency spectrum occupied by existing radio services without causing harmful interference, thereby permitting scarce spectrum resources to be used more efficiently.
Having recognized the promising benefits and potential for harmful interference associated with UWB technology, governmental agencies in various parts of the world have begun cautiously considering and allocating portions of the frequency spectrum for unlicensed use by UWB devices. For example, the FCC amended Part 15 rules to permit unlicensed operation of UWB devices. In April 2002, the FCC released a First Report and Order in connection with Part 15 revisions (In re: Revision of Part 15b of the Commission's Rules Regarding Ultra-Wideband Transmissions systems (ET Docket 98-153), FCC 0248 document, which is hereby incorporated by reference.)
At the present time, work is in progress for developing standards for exploiting UWB technology. One such effort is by an IEEE working group under IEEE 802.15. Information for obtaining all published documents of the IEEE standard setting body may be obtained by visiting the IEEE website, www.ieee.com. Briefly, these UWB standards will apply to UWB devices operating in shared or in non-government frequency bands, including UWB devices operated by U.S. Government agencies. In general, the FCC rules establish corresponding technical standards and operating restrictions for various types of UWB devices mainly based on their potential to cause interference. For example, outdoors use of UWB devices are currently restricted to certain imaging systems, hand held devices, and vehicular radar systems that operate with very low power. In fact, UWB devices with potential for use in high power applications, such as wide-area mobile radio services, are not permitted to operate.
The rules governing operational restrictions are generally specified in terms of allocated spectrum, minimum bandwidth, and emission limitations for each UWB device type. The rules divide the frequency spectrum into sub-spectrums, with each sub-spectrum being subject to corresponding operational and emission limitations based on the type of the UWB device. For example, ground penetrating radar (GPR) and wall imaging systems are permitted to operate in the 3.1-10.6 GHz frequency band and through-wall imaging systems are permitted to operate in the 1.99-10.6 GHz frequency band. Surveillance systems are permitted to operate in the 1.99-10.6 GHz frequency band, and medical systems must operate in the 3.1-10.6 GHz frequency band. Communication devices are permitted to operate in the 3.1-10.6 GHz frequency band.
The frequency band of operation of UWB devices is based on the −10 dB bandwidth of the UWB emission. For example, the FCC rules define a UWB device as any device where the fractional bandwidth is greater than 0.20 or has a minimum bandwidth of 500 MHz, i.e., occupies 500 MHz or more of spectrum.
At least initially, the adopted rules are significantly stringent. However, the FCC has acknowledged that the initial rules are extremely conservative and may change in the future as more and more data is collected regarding UWB emissions. For example, in order to limit unwanted emissions from UWB devices, the FCC has initially adopted more conservative limits than those imposed on other Part 15 devices. Taking into account lack of experience with UWB devices, FCC rules regarding UWB emission limits are defined in terms of a reduction to the Part 15 general emission levels over defined frequency bands to ensure that UWB devices have the least possible impact to authorized radio services. Moreover, the emission limits are also designed to ensure that harmful interference from the cumulative effect of multiple UWB devices is minimized.
The following table specifies the average emission limits in terms of dBm EIRP as measured with a one megahertz resolution bandwidth for UWB operation.
ImagingImaging,Imaging,Hand held,Frequencybelow 960Mid-HighIndoorincludingVehicularBand (MHz)MHzFrequencyfrequencyapplicationsoutdoorradar0.009-960  §15.209§15.209§15.209§15.209§15.209§15.209 960-1610−65.3−53.3−65.3−75.3−75.3−75.31610-1990−53.3−51.3−53.3−53.3−63.3−61.31990-3100−51.3−41.3−51.3−51.3−61.3−61.3 3100-10600−51.3−41.3−41.3−41.3−41.3−61.310600-22000−51.3−51.3−51.3−51.3−61.3−61.322000-29000−51.3−51.3−51.3−51.3−61.3−41.3Above 29000−51.3−51.3−51.3−51.3−61.3−51.3
FCC rules also allow for the use of various forms of modulation as long as the UWB devices comply with all of the technical standards defined by the rules. Thus, as long as the transmission system complies with the fractional bandwidth or minimum bandwidth requirements at all times during its transmission, it is permitted to operate under the UWB regulations. It is up to the manufacturers of UWB devices to determine how they will comply with the UWB standards.
The above-mentioned combinations of technical standards and operational restrictions are designed to ensure that UWB devices coexist with licensed radio services without the risk of harmful interference. Clearly, the FCC-promulgated rules are extremely complex, applying numerous detailed standards and restrictions to different types of UWB devices based on their potential to cause harmful interference. In particular, FCC rules have been tailored to protect sensitive portions of the US spectrum from possible UWB interference, e.g., the global positioning service (GPS) band. These requirements for different UWB emission levels at different portions of the spectrum in effect creates a “frequency mask” to which UWB devices must be designed.
Additionally, other countries have their own sets of detailed and complex spectrum management rules. UWB emission standards established by regulating agencies in other countries will likely have provisions tailored to protect sensitive portions of their spectrum. Because spectrum allocation and emission standards vary in different regions of the world, UWB rules and restrictions are also likely to vary substantially from region to region. In other words, each country or region will have its own frequency mask. Furthermore, the various frequency masks established for the various countries and regions are subject to change over time.
Making matters even more complicated is the desire by some manufacturers to protect investments in systems that operate under already defined IEEE standards discussed above, which are not subject to FCC Part 15 rules. Indeed, in some cases, UWB emissions restrictions imposed by such non-FCC standards may be even more stringent than FCC requirements. Such non-FCC-imposed restrictions must also be taken into account when designing UWB devices. Accordingly, because frequency masks to which UWB devices must be designed will vary from country to country and because these masks are subject to change as government and industry emission standards evolve, there is a need for UWB devices that have the flexibility to vary their emissions to meet the various UWB spectral requirements.
Moreover, the cost of UWB devices is increasingly becoming a critical factor as the use of wireless devices permeates to create a consumer base that constantly strives for smaller devices having long battery life. The cost concern becomes even more prevalent if multi-national manufacturers are to maintain multiple inventories of devices that cover different applications and meet country-specific emission requirements. Therefore, there exists a need for UWB devices capable of efficiently and cost effectively operating under the various frequency masks.
Generally, RF transmission of information requires the creation of an RF carrier in the transmitter that is modulated with the information. RF reception requires “mixing” the incoming modulated carrier for demodulation and recovery of the transmitted information. Known narrowband systems use fixed-frequency sources for transmission and reception. However, UWB systems require sources that generate frequencies at a rate on the order of the information rate. One known UWB system is disclosed in the U.S. Pat. No. 6,026,125, entitled “Waveform Adaptive Ultra-Wideband Transmitter,” issued to Larrick et al. Other prior art methods use fast switching phase locked loops (PLLs), super heterodyne frequency shifting, and frequency-tracking filters for generating fixed frequencies at a high rate. However, the implementation of such methods requires complex circuitry, with limitations that makes them difficult to build on integrated circuits (ICs). A fast-switching PLL needs a low-jitter voltage-controlled oscillator (VCO) that can be tuned to a new frequency very quickly. Generating low-jitter VCO output at high rate is not easy to accomplish and leads to complex, power consuming circuitry. Moreover, highly linear mixers with wide dynamic ranges are needed for super heterodyne frequency conversion. Such linear mixers require precision components to achieve balanced operation, which makes them difficult to integrate. Frequency-tracking filters, preferably, containing tunable passive or reactive components, (e.g., capacitors and inductors), are also needed to change the frequency of operation. It is well known that passive or reactive components are also difficult to integrate. The requirement for rapidly tunable filters further complicates integration.
Primarily, integrated circuit implementation that meets the above-described requirement should be simple to implement, inexpensive to produce, and consume as little power as possible. Consequently, it is necessary to reduce the number of components, particularly those components that are difficult to integrate with other active circuitry. Further, the cost effective circuit integration requires accounting for component variations both across a single IC as well as from batch to batch.